Two hours before the blasts, racing in only his second marathon, the 23-year-old Ethiopian Lelisa Desisa crossed the finishing line in first place, with a time of two hours, 10 minutes and 23 seconds.
It had been an excellent morning for marathon running in Boston: clear and dry, warm but not hot. A few blocks away in Fenway Park, the Red Sox were about to beat the Tampa Bay Rays. Behind Mr Desisa were another 23,000 runners, and around them some half a million spectators. Among those watching, in the VIP stands close to the finishing line, were the families of those killed in the Newtown shootings in December.
Boston's is the world's oldest marathon, first run in 1897 and always held on Patriots' Day, the third Monday in April, when the states of Massachusetts and Maine remember the battles of Concord and Lexington on 19 April 1775: the opening salvos of the American Revolutionary War, which Ralph Waldo Emerson described as "the shot heard round the world".
But suddenly at 2.50pm local time on Monday two explosions tore through shopfronts on the north side of Boylston Street, around 100ft from the end of the marathon's course. They were met first with stunned silence, and then with screams. Spectators scrambled to escape as smoke rose 50ft into the air. The blasts came as the course was still clogged with amateur runners, one of whom, 78-year-old Bill Iffrig, was thrown to the ground by the shock wave.
John Mixon, of Ogunquit, Maine, was knocked from his perch on the bleachers at the finish line. A Vietnam veteran, Mr Mixon told the Portland Press Herald he could smell explosives, and knew immediately it was a bomb attack. Mr Mixon had been watching the race with 52-year-old Carlos Arredondo, a peace activist whose son, Alexander, was killed in action in Iraq in 2004. Rather than flee, Mr Mixon and Mr Arredondo ran towards the carnage, fighting their way through the tangled crowd control barriers to help the wounded.
"When we got over there, it was a pile of bodies – people with legs missing," Mr Mixon said later. "It was absolutely like a war scene. This was worse, because it was all innocent people, just defenceless. They were lying in a pile, gunpowder all over them, burnt."
Roupen Bastajian, 35, a state police officer and another former Marine, had just finished the race and was wrapping himself in a heat blanket when he heard the blasts. He, too, ran towards the sound. The scene of the explosion was chaotic and bloody, he told the BBC.
"There were people all over the floor… We started grabbing tourniquets and started tying legs. At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
The first-aid tent at the finishing line had been set up to treat exhausted runners, but it quickly became a field hospital. Doctors and nurses who had taken part in the race shook off their fatigue and offered their services. For Mr Iffrig, meanwhile, there was only one thing to do. After dusting himself down with the help of a concerned race official, and deciding he had suffered no more than cuts and bruises, the defiant 78-year-old walked the final few feet to the line. His time? Four hours, three minutes and 47 seconds. "After you've run 26 miles, you're not going to stop there," he said.
By five o'clock, as Bostonians generously opened their homes to those stranded in the city centre, the authorities were already deep into their investigation. The Boston Police Commissioner Edward Davis told a press conference he wasn't ready to call the explosions a terrorist attack, but said, "You can reach your own conclusions based upon what happened."
The media did just that, rushing to link the blasts to a fire that broke out at Boston's John F Kennedy Library an hour later, and which turned out to be unrelated. Rumours of three other unexploded devices around the city also proved to be false. A Saudi national detained near the scene was cleared of any involvement in the attack. The young man, reportedly a student, had suffered injuries in the explosion.
At 6.10pm, President Barack Obama made a televised address, warning that "people shouldn't jump to conclusions before we have all of the facts. But make no mistake, we will get to the bottom of this and we will find out who did this... Any responsible individuals, any responsible groups will feel the full weight of justice."
As the fog of confusion began to lift on Monday evening, three people were confirmed dead. One of them was eight-year-old Martin Richard, from Ashmont, Massachusetts, who had been waiting to see his father, William, cross the finishing line. Martin's mother underwent surgery for a brain injury. His six-year-old sister lost a leg in the blast.
Boston's hospitals said they were treating at least 176 people for their injuries, at least 17 of whom were in a critical condition. Peter Fagenholz, a trauma surgeon at Massachusetts General Hospital, told reporters that many of the injuries were caused by shrapnel from the bombs, and that several patients had had to undergo amputations. Two wounded brothers from the Boston suburb of Wakefield were taken to separate hospitals, where each lost a leg.
When President Obama took to the airwaves again this morning, he confirmed the attacks were now being treated as terrorism. "This was a heinous and cowardly act and, given what we now know about what took place, the FBI is investigating it as an act of terrorism," he said. "Any time bombs are used to target innocent civilians, it is an act of terror."
At the scene of the explosions in Boylston Street, the marathon detritus remained long after the streets were cordoned off and cleared of people: rows of water bottles, stacks of heat blankets, unclaimed bags filled with each runner's change of clothes. The banner still flapped in the breeze above the finishing line. Of the 23,000 people who started the marathon on Monday, only 17,580 reached it.
Amid the carnage: Images of resilience
Most of the images to come out of the tragic events at the Boston marathon on Monday showed in stark detail the result of an inhumane act.
But there were many that did the opposite.
One man who appears in a number of pictures taken in the immediate aftermath of the attack has been hailed as a hero after his story was revealed.
The cowboy-hatted Costa Rican immigrant Carlos Arredondo was photographed coming to the aid of a man who had lost both his legs in the explosion.
Mr Arredondo can be seen seemingly pinching shut the end of an artery on the part-severed leg of a man.
The victim was later reported to be in a stable condition.
Mr Arredondo, 52, was waiting at the finishing line for a runner who was racing in memory of his son, Alexander Arredondo, a lance corporal in the US Marines who was killed by a sniper in 2004 in Iraq.
Mr Arredondo became a travelling peace campaigner after his son's death – driving across the country to share his message with others.
Another man whose picture went around the world was Bill Iffrig, a 78-year-old veteran of 45 marathons.
He was knocked off his feet by the blast of the first bomb, and the photo of the dazed and confused man being helped to his feet has quickly become one of the defining images of the attacks.
"The shock waves hit my whole body and my legs started jittering around.
"I knew I was going down and so I ended up down on the blacktop [the asphalt]," he told CNN.
Mr Iffrig suffered only minor injuries, and made it across the finishing line.